(1) Field of the Invention
The present invention relates to the field of methods of regulating the operation of the engines in a power plant of a multi-engined rotorcraft. Said power plant comprises in particular main fuel-burning engines, in particular turboshaft engines, that conventionally supply the rotorcraft with the mechanical power needed at least for driving one or more rotors of the rotorcraft.
The present invention lies more particularly in the context of a failure of one of said main engines of the rotorcraft serving to drive at least one main rotor of the rotorcraft at a setpoint speed that is variable, and possibly also serving to drive an anti-torque rotor.
(2) Description of Related Art
Typically, the main rotor provides the rotorcraft at least with lift and possibly also with propulsion and/or its ability to change flight attitude in the specific example of a helicopter. Typically, the anti-torque rotor serves to stabilize the rotorcraft and to guide it in yaw, and it is commonly formed by a tail rotor, or else by at least one propulsive propeller for a rotorcraft having high forward speeds.
Conventionally, the operation of the main engines of a rotorcraft is placed under the control of a full authority digital engine control (FADEC) regulator unit. The regulator unit controls the supply of fuel to the main engines as a function of a setpoint, referred to below as the NR setpoint, relating to a speed of rotation required for the main rotor. The NR setpoint is generated and is transmitted to the regulator unit by a control unit, such as an automatic flight control system (AFCS).
The NR setpoint is commonly generated by the control unit as a function of the mechanical power needs of the rotorcraft, which needs are identified depending on the current flight situation of the rotorcraft, and in particular as a function of mechanical power needs for driving the main rotor. The power consumed by the main rotor may be identified, by way of example, on the basis of evaluating the resisting torque that the main rotor opposes to being driven by the power plant.
It is known to perform calculations for the purpose of anticipating the mechanical power that the power plant is going to need to deliver in order to satisfy the needs of the rotorcraft, so as to ensure that the main rotor is quickly driven at a speed of rotation that matches the NR setpoint. The calculation of the mechanical power needed by the rotorcraft in anticipation may be based on flight control signals issued by a pilot of the rotorcraft, which pilot may equally well be a human pilot or an autopilot.
The flight control signals used to calculate the mechanical power needed by the rotorcraft in anticipation comprise in particular control signals for varying the pitch of the blades of the main rotor, at least collectively, and possibly also cyclically. When the rotorcraft has a tail rotor, that rotor may also be taken into account in the event of a flight control signal leading to a variation in the pitch of its blades, in particular collective variation, when calculating in anticipation the mechanical power that is going to be needed by the rotorcraft.
In this context, there arises the problem of failure of one of the main engines of a twin-engined rotorcraft, or of a plurality of main engines of a rotorcraft that has more than two main engines. Under such circumstances, only one of the main engines of the rotorcraft might remain operational in order, on its own, to supply all of the mechanical power needed by the rotorcraft.
That is why specific power ratings have been defined for regulating the operation of main engines, commonly known as one engine inoperative (OEI) power ratings. OEI power ratings are applied to regulate the operation of a main engine supplying on its own the mechanical power needed by a rotorcraft in flight in the event of at least one other main engine of a multi-engined rotorcraft failing. OEI power ratings are typically defined for specific stages of flight in terms of a given mechanical power level that is to be supplied for a given period by the main engine while ensuring that it is not damaged beyond an acceptable damage threshold.
Various OEI power ratings can potentially be applied by the regulator unit, either automatically (by means of a controller) or else at the request of the human pilot of the rotorcraft in compliance with the flight manual. The following OEI power ratings are commonly defined:
a very short duration OEI power rating in which the operational main engine(s) may be used individually at an emergency power rating for a duration that is brief, of the order of 30 seconds;
a short duration OEI power rating in which the operational main engine(s) may be used individually at an emergency power rating for a duration that is short, of the order of 2 minutes to 3 minutes; and
a long duration OEI power rating, in which the operational main engine(s) may be used individually at a maximum power rating for a duration that is long, and potentially unlimited.
The NR setpoint is defined by the control unit so as to obtain a speed of rotation of the main rotor that is referred to below as the speed NR.
The speed NR is traditionally predefined as being substantially constant, being allowed to vary, depending on the flight attitude of the rotorcraft, over no more than a narrow range of speed variation of the order of 5% of a nominal speed NR, while nevertheless not exceeding variation of the order of 1% per second. The impact of such narrow variation on the speed NR is negligible on varying the mechanical power to be supplied by the main engines of the rotorcraft in order to drive the main rotor.
A failure of one of the main engines of a rotorcraft leads to a sudden loss of mechanical power that can be supplied by the power plant, and consequently leads to a drop in the speed NR. Nevertheless, at the instant one of the main engines of the rotorcraft fails, the current speed NR is substantially equal to the current NR setpoint and is still sufficient to enable the pilot to control the attitude of the rotorcraft in conventional manner.
More particularly, and with reference to FIG. 1 of the accompanying sheets, there can be seen a diagram showing how essential events that occur when one of the main engines of a twin-engined rotorcraft fails vary relative to time (t).
In a first step shown E1, both of the main engines of the rotorcraft are operational and, except in a situation of one of the main engines failing, referred to as an engine-failure situation PM, they act together to supply mechanical power PU1 that serves to drive the main rotor at a nominal speed NRnom that is substantially constant for a current given pitch P1 of the blades of the main rotor. In such a context, the speed NR at which the main rotor is driven can nevertheless vary, for a given current pitch P1 of the blades of the main rotor over a range having a value of about 5%, and conventionally extending from 97% to 102% of the nominal speed NRnom.
In the event of an engine-failure situation PM in which one of the main engines fails, the rotorcraft is suddenly placed in a second step E2 before the pilot, who is suddenly confronted with an engine-failure situation PM, has any time to react. An OEI power rating for regulating the sole main engine that is still in activity is immediately put into operation. The OEI power rating that is applied is conventionally selected automatically from a set of OEI power ratings that are applicable depending on the flight stage of the rotorcraft as characterized by its flight mechanics and its travel conditions such as its attitude, its progress altitude, and/or its forward speed, for example, and the selected OEI power rating is put into operation.
During this second step E2, a sudden drop in the available mechanical power PU2 arises as a result of one of the main engines failing, and the speed NR of the main rotor drops given that the current pitch P1 of the blades of the main rotor has remained unchanged in the absence of any reaction from the pilot.
The main engine that continues to be operational is then subjected to acceleration, which has the effect of increasing the mechanical power PU3 it supplies in compliance with the OEI power rating that has been selected and put into operation by the regulator unit. Naturally, it should be understood that the term “main engine that continues to be operational” is used to mean the main engine of the rotorcraft that is operating, as contrasted to the main engine that has failed.
Then in a third step E3, the pilot reacts in order to vary the current collective pitch P2 of the blades of the main rotor, seeking to reduce the power required by the main rotor. Such a variation in the collective pitch P2 serves progressively to limit and then to stop as quickly as possible the drop in the speed NR of the main rotor, before it reaches a critical threshold speed, referred to below as the target speed NRobj, having a constant value of the order of 97% of the nominal speed NRnom.
In a fourth step E4, the main engine that continues to be operational supplies mechanical power PU4 in compliance with the OEI power rating that enables the main rotor to be driven at the substantially constant nominal speed NRnom.
With the lift of the rotorcraft stabilized in spite of the failure of one of the main engines, the pilot can make use of all of the flight controls in order to place the rotorcraft in a stabilized flight situation, enabling the overall behavior of the rotorcraft to be stabilized, while conserving as well as possible the drive speed NR of the main rotor constant at the nominal speed NRnom. Such a stabilized flight situation is commonly recognized as being achieved when the flight parameters of the rotorcraft cease varying while the rotorcraft is making stable progress.
In order to specify in greater detail the concept of stabilized lift, a rotorcraft is commonly recognized as having lift that is stabilized when the drop in the number of revolutions per minute of the main rotor comes to an end, providing the rotorcraft is safe relative to withstanding the forces to which its structure is being subjected, it naturally being understood that said drop in the number of revolutions per minute takes place independently of the behavior of the rotorcraft being controlled by the pilot, whether a human pilot or an autopilot, as in the event of detecting that one of the main engines of the rotorcraft has failed in the context of the present invention.
Such pilot interventions as shown in FIG. 1 are conventionally performed in compliance with flight manuals in the situation of the main rotor being driven at a speed NR that is substantially constant and that is considered as being invariable.
Depending on the equipment of the rotorcraft, it is possible that an autopilot is used to cause the nominal speed NRnom of the main rotor to be reestablished rapidly in the event of one of the main engines failing, by generating automatic flight control signals for modifying the current collective pitch P2 of the blades of the main rotor, as illustrated by the third step E3 shown in FIG. 1.
Nevertheless, technical changes in the field of rotorcraft are tending to encourage the main rotor being driven at a controlled speed NR that is variable relative to the nominal speed NRnom as predefined depending on the flight conditions of the rotorcraft.
By way of example, such a significant variation in the drive speed NR of the main rotor may be used in order to reduce the sound nuisance of the rotorcraft and/or in order to improve its performance during certain stages of flight. By way of indication, the speed of the main rotor may be controlled so as to variable over a range of 5% to 10% of the nominal speed NRnom, and possibly over a larger range depending on technical changes, and more particularly it may be controlled to vary over a range of values potentially lying from 93% to 107% of the nominal speed NRnom.
On this topic, reference may be made for example to the publication “Enhanced energy maneuverability for attack helicopters using continuous variable rotor speed control” (C. G. Schaefer Jr., F. H. Lutze Jr.); 47th Forum American Helicopter Society 1991; pp. 1293-1303. According to that document, the performance of a helicopter in a combat situation is improved by varying the drive speed of the main rotor depending on variation in the air speed of the rotorcraft.
Reference may also be made, for example, to the Document U.S. Pat. No. 6,198,991 (Yamakawa, et al.), which proposes reducing the sound nuisance generated by a rotorcraft approaching a landing point by varying the speed of rotation of the main rotor.
On this topic, reference may also be made, by way of example, to the Document US 2007/118254 (G. W. Barnes, et al.), which proposes varying the speed of rotation of the main rotor of a rotorcraft between two values referred to as “low” and “high”, under predefined threshold conditions for the values of various parameters associated with the previously-identified flight conditions of the rotorcraft.
Document EP 2 724 939 describes a method of managing an engine failure on a rotary wing aircraft having a hybrid power plant with at least two fuel-burning engines. That method enables the pilot to maneuver the aircraft with engine power but without stressing the engine that remains operational. An electronic control unit EECU is connected to each engine, and electrical energy storage means power an electric motor so that the main rotor is driven by the hybrid power plant.
Document FR 2 900 385 describes a method of piloting a rotorcraft that has a plurality of engines for driving at least one advance and lift rotor. In that method, so long as the rotorcraft has not reached an optimum climb rate, a pitching control signal is determined so that the rotorcraft accelerates with a profile that varies during takeoff, firstly as a function of elapsed time and secondly as a function of the operating state of the engines.
Also by way of example, reference may be made on this topic to Document WO 2010/143051 (Agusta S P A, et al.), which proposes varying the speed of rotation of a main rotor of a rotorcraft in compliance with a map previously established for various flight conditions of the rotorcraft.
There then arises the problem of how to intervene on the behavior of the rotorcraft in the event of one of the main engines failing, given that the main rotor might then be being driven at a speed NR that is low relative to the nominal speed NRnom, and possibly as much as 7% less than the nominal speed NRnom. Under such circumstances, it is much more difficult for the pilot to reestablish drive of the main rotor at a speed NR complying with the NR setpoint.
Consequently, it appears appropriate to provide the human pilot of a twin-engined rotorcraft with automated assistance for reestablishing drive of the main rotor in the event of one of the main engines failing, in the context of it being possible that the main rotor is being driven at a speed NR that is low relative to the nominal speed NRnom at the instant when said one of the main engines fails.
A technological environment of the invention as applied to a single-engined rotorcraft is known, in which automated assistance is provided for the human pilot of the rotorcraft in order to place the main rotor in auto-rotation in the event of the main engine failing.
Such assistance is provided by an automatic device that generates flight control signals acting in the event of the main engine failing to modify the attitude of the rotorcraft, vertically, in pitching, in roll, and/or in yaw, in order to counterbalance the unfavorable aerodynamic effects that occur immediately after a failure of the main engine.
By way of example, reference may be made on this topic to the following documents: FR 2 601 326 (United Technologies Corporation); FR 2 864 028 (Eurocopter SAS); and US 2013/0221153 (Bell Helicopter Textron).